MUSCULAR SYSTEMMonday, November 05, 201212:05 PM Muscular System:-contains muscles as organs-3 types of muscle tissues:1.)skeletal muscle tissue2.)cardiac muscle tissue3.)smooth muscle tissue-muscle tissues are wrapped in connective tissue membranes with blood vessels and nerves*this structure will form muscles which is organsSkeletal Muscle:STRUCTURAL ORGANIZATION OF SKELETAL MUSCLE (THE ORGAN)-each skeletal muscle fiber-> wrapped by the endomysium-fascicle= wrapped by the perimysium-a bundle of perimysium covered fascicles epimysium forms skeletal muscle= the organ-skeletal muscles attach to bones in the skeletal system-attachment onto a bone leads too:1.)direct attachment= the skeletal muscle attaches directly with the epimysium to the surface of a bone2.)indirect attachment= the epimysium of a skeletal muscle is extended by a tendon*the tendon attaches the skeletal muscle to the surface of the bone-with the exception of skeletal muscles attaches to the skull, skeletal muscles in the human body are attached indirectly (601 named skeletal muscles in the human body) –advantages of indirect attachment of skeletal muscles:1.)packaging-> the tendon occupies a smaller area on the surface of a bone compared to the skeletal muscle attaching directly-more than one skeletal muscle can be attached to the same bone2.)since the surface of most bone is rough, direct attachment of skeletal muscle may result in damage tot eh skeletal muscle-> in indirect attachment the skeletal muscles do not come into direct contact with the surface of the bones3.)bones act as levers when skeletal muscle indirectly attached contract pulling on the bones to course movements-each skeletal muscle fiber->contains*myofibrils->contain-myofilaments= thick and thin filaments-the thick filaments alternate with the thin filaments-> alternating pattern is responsible for the striated appearance of skeletal muscle-mitochondria-> for the aerobic cataboal of nutrients to produce energy required for skeletal muscle contraction-myoglobin= red pigment that binds and releases oxygen for aerobic respiration to produce energy-specialized smooth ER called sarcoplasmic reticular= SR= the expanded ends of SR= terminal cisternae SR= stores/releases calcium-cytoplasm of the skeletal muscle fiber= sarcoplasm-plasma membrane= referred to as sarcolemma (imaginations: in-folding) of the sarcolemma are called transverse tubules= T-tubules-each T-tubule goes through 2 terminal cisternae of adjacent SR-each skeletal muscle fiber with alternating thick/ thin filaments, the thin filaments are anchored by the Z discs*the distance between successive Z-discs= sarcomere-sarcomeres run the entire length of a skeletal muscle fiber-skeletal muscle fibers run the entire length of skeletal muscle the organ*hence, the structural units of skeletal muscles are the sarcomeres-a sarcomere is the smallest contractile unit in a skeletal muscle*this means we need to explain what skeletal muscles contain at that levelSARCOMERES:-distance between two successive Z-discs-in the center of 6 sarcomere-> thick filaments are referred to as A bands-> with the myosin globular headexposed-alternating with the A band-> thin filaments are anchored by the Z-discs-the region/ regions of the thin filaments not overlapping with the A bands are called the I bands-I bands are attached to the Z discs Endomysium - wraps muscle fiberPerimysium - wraps group of fascicleEpimysium - wraps fascicle Sliding filament Mechanism of Muscle Contraction-Sliding of the thin filaments past the A bands results in muscle contractionH zone decrease/disappearI bands decrease/disappearSarcomere length shortens (skeletal muscle contracts/shortens) --> Skeletal muscle contraction (shortening) is explained by the sliding filament mechanism --> sarcomeres shorten when the thin filaments SLIDE into H-zone (toward M-line). Since skeletal muscle (organ) fiber is composed of sarcomere, the skeletal muscle fiber shortens & skeletal muscle shortensA-bands (thick filaments) & thin filaments - do not shorten Excitation-Contracting CouplingSteps:-Motor neuron makes contact w/ skeletal muscle fibers (axon terminals)Axon terminal & 1 muscle fiber = neuromuscular junctionMotor Unit --> Activation or excitation of a motor neuron is coupled to contraction of skeletal muscle.Each skeletal muscle containing several skeletal muscle fibers is innervated by a MOTOR NEURON(in somatic nervous system)Motor neuron makes contact w/ skeletal muscle fibers (through axon terminals)Each skeletal muscle fiber makes contact w/ 1 axon terminal --> forms NEUROMUSCULAR JUNCTION(Each skeletal muscle fiber can form only one neuromuscular junction)The motor neuron & all skeletal muscle fibers of innervates via axon terminals --> MOTOR UNITSkeletal muscle (organ) contains several motor unit of varying sizes (small (innervates 10 skeletal musclefibers), medium (20), large (50-100))Neuromuscular junction forms specifically between the axon terminal and the MOTOR END PLATE (high convoluted area of the sarcolemma of skeletal muscle fiber)Motor End Plate - expresses the acetylcholine receptorsE-C Coupling (Sequence of Events): (Electrical Signal --> Chemical Signal --> Electrical Signal)1. Activate motor neuron (axon generates & transmits action potentials to axon terminal)2. Neurotransmitter (Acetylcholine released from vesicles (in axon terminals) into neuromuscular cleft (small fluid-filled space b/t axon terminal & motor end plate))3. Acetylcholine diffuses from neuromuscular cleft to bind to acetylcholine receptors on motor end plate--> Motor end plate depolarizes & action potential is generated & transmitted across entire sarcolemma (T-tubules in the triads)4. Action potential traveling through T-tubules depolarizes the terminal cisternae of the SR & ionic calcium is released into sarcoplasm 5. Ca2+ binds TnC (of troponin)--> Causes a conformed change that will resort in the removal of tropomyosin from blockside, the myosin binds sites on actin (hence, Ca2+ ends the tropomyosin blockade on actin --> myosin binding sites on actin are exposed)Picture p. 292 (textbook)(Only activated myosin globular bands can bind to the exposed myosin binding sites on actin)ATP bound to myosin globular head
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